Fix testIncludesWithNullElement: exclude main artifact to avoid duplicate

src/test/java/org/apache/maven/plugins/jarsigner/JarsignerVerifyMojoTest.java

diff --git a/src/test/java/org/apache/maven/plugins/jarsigner/JarsignerVerifyMojoTest.java b/src/test/java/org/apache/maven/plugins/jarsigner/JarsignerVerifyMojoTest.java
index 107bc47..b3ff2a3 100644
--- a/src/test/java/org/apache/maven/plugins/jarsigner/JarsignerVerifyMojoTest.java
+++ b/src/test/java/org/apache/maven/plugins/jarsigner/JarsignerVerifyMojoTest.java
@@ -175,6 +175,7 @@
         when(project.getArtifact()).thenReturn(mainArtifact);
         when(jarSigner.execute(any(JarSignerVerifyRequest.class))).thenReturn(RESULT_OK);
         configuration.put("archiveDirectory", dummyMavenProjectDir.getPath());
+        configuration.put("processMainArtifact", "false");
 
         JarsignerVerifyMojo mojo = mojoTestCreator.configure(configuration);